Very high CO2 cordierite from Norwegian Lapland: Mineralogy,petrology, and carbon isotopes

The most CO₂-rich cordierite thus far encountered in nature with about 2.2 wt.% CO₂ and 0.3 wt.% H₂O occurs as large poikiloblasts in a strange non-foliated reaction rock that dissects well-foliated granulites being part of the classical Lapland granulite area described by Eskola. The cordierite is optically positive with the highest optic angle 2V_x (106°) and birefringence (– = 0.017) ever measured on natural cordierites, but it is also optically very heterogeneous due to secondary loss of CO₂ along fractures and zones paralleling the fluid-bearing channels. Based on the optical properties of the degassed Lapland cordierite and on literature data a ternary diagram is given, which shows the variations of this cordierite in 2V_x and birefringence as a function of channel-filling with both CO₂ and H₂O.Following Losert (1971) the cordierite coexists with calcite, a thus far unique mineral assemblage that is probably only stable at very high CO₂ pressures. In the present case, the of the cordierite (0.75) indicates, on the basis of literature data, a coexisting fluid with >0.95.The carbon isotope composition ¹³C of CO₂ in cordierite lies near –7, that of the calcite is slightly lighter than about –9. Thus, at least for the CO₂ in cordierite, a deep-seated origin may be possible.Based on the geologic occurrence it is speculated that the cordierite-bearing reaction rock could perhaps represent an annealed channel of late degassing in the granulitic lower crust.